Han Raven

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“My key interest is understanding faunal communities living in different depositional environments and how these change through geological time”

In my teenage years I became passionate about natural history, with a special interest in shells. The field work during my geology study took me to different parts of Spain where I collected terrestrial snails and discovered the impact of geology and climate on their distribution. My geogical studies and thesis focused on the stratigraphy and depositional history of the Devonian and early Carboniferous of the Cantabrian mountains (NW Spain), in which I used conodonts as indicator for age.

I became an active member of the Nederlandse Malacologische Vereniging and the Werkgroep voor Tertiaire en Kwartaire Geologie studying NW European faunas: from the tropical faunas in the Eocene of the Paris Basin to the temporary outcrops with Holocene faunas and recent ones.

Working for Sarawak Shell Berhad I lived six years in Sarawak (Malaysia) during which I sampled and studied local miocene to recent faunas and had an opportunity to travel extensively through the region. The depositional environments in the region and their faunas have undergone many changes. Especially the large changes during the Holocene sealevel rise are remarkable. There is sufficient material for many years of further study. I am keen to cooperate with specialists on specific mollusc groups.

Career

Since 1984 I work for Shell International, initially as production geologist, currently as General Manager Venture Planning & Implementation.

At Naturalis Biodiversity Center I hold a position as research associate.

I have been board member of various natural history societies: Werkgroep voor Tertiaire en Kwartaire Geologie, Nederlandse Malacologische Vereniging (Netherlands) and Miri Natural History Society and helped to set up the Sarawak Branch of the Malaysian Natural History Society (Malaysia).

Research

Research interest

Keywords

Current research topics

MOLLUSCS FROM BORNEO

During the period (from 1992-1997) I lived in Miri, Sarawak I had the opportunity to study molluscs living in a wide range of environments and also Neogene sediments with their mollusc faunas. Although I was working as geologist, these studies were done on personal title in my spare time. Based on the material collected (of which thus far only a small part has been studied in detail) many interesting observations have been made. This page intends to present some of these insights (some have been published – see list of publications) and to invite others to collaborate on the study of the rest of the material – which is too wide ranging for a single individual to review in sufficient detail. The page is also meant to provide access to some of the available resources. If you have comments on this page or are interested in collaboration on a specific subject please contact me on schelp56@hotmail.com.

INTRODUCTION

The area between the Asian mainland and Australia comprises a huge number of islands, many of which form part of Indonesia and the Philippines. This area separates the Indian and Pacific Oceans which besides a large number of common faunal elements also have species that only live in one of these oceans or a much smaller area. One of the largest islands, Borneo, comprises an Indonesian part (Kalimantan, in the South), two Malaysian states (Sarawak and Sabah, forming East Malaysia in the NW) and Brunei Darussalam (a separate country, in between Sarawak and Sabah). The author's field work focused on the NW part of the island bordering the South China Sea (Sarawak, Brunei and Sabah) but on this page some reference will be made to the other parts of the island.

DEPOSITIONAL ENVIRONMENTS

As geologist with strong interest in biology I look at different biotopes as parts of depositional systems, which may form different facies in the geological record. The terminology used by geologists is often not or poorly understood by non-geologists, therefore a short description of key depositional environments from NW Borneo is given below:

Terrestrial. Until recently, except from the coastal strip, most of Borneo was quite inaccessible and covered by primary rainforest. Terrestrial snails are not easy to collect as many live on the leaves high in the trees. Empty shells can be found on the ground between the leaves. Where the rainforest has been cut often dense secondary vegetation (“jungle”) appears which is quite impenetrable. It is easiest to find shells where this vegetation has been burnt but typically the shells will be damaged by the fire.

More varied mollusc faunas can be collected in rocky areas, especially on limestone rocks. There is a huge variation in small species some of which have developed very special shapes, often in defence to their predators. Some of these occur only in very small areas, sometimes only a single limestone hill. As many of these hills are excavated by the xx industry several species are threatened.

The shells of terrestrial molluscs are typically thin and therefore may dissolve quickly upon their death. I am not aware of fossil terrestrial molluscs besides a few from the Holocene. Those from the archaeological record of the Niah caves (Sarawak) are currently being studied.

Fluviatile

There are numerous rivulets and rivers in NW Borneo and a small number of natural lakes. Their mollusc faunas have not yet been studied systemically. I have collected some samples but am certain I have only seen a very small part of what is available.

Mixed salinities

Estuaries form at river mouths. Depending on the type and size of river, currents and wind estuaries may take very different shapes. What they have in common is that fresh and salt water mix creating an environment with brackish water and/or widely varying salinities. At the same time, due to the change in salinity clay minerals that are in suspension in the river water will flocculate and deposit. Estuaries therefore are typically muddy and nutrient rich environments. Relatively few species have adapted to these special circumstances but typically they occur in large numbers.

These adapted species also include plants. Grass will grow both within the intertidal range (typically short) and on low lying areas that only get flooded during the strongest tides (typically long) called marshes. Very few molluscs survive in this environment.

Certain trees have adapted to the life in estuaries and can form forests within and along the edges of estuaries. These are named after their most noticeable group of representatives- the mangroves. Mangrove trees form stilts to which oysters and gastropods may attach or into which Pholadidae and Teredinidae may bore, whereas Littoraria lives in the leaves. Another key representative in the nipa palm which also hosts a wide variety of molluscs.

Tidal flats form in areas that are (partially) shielded from waves by islands, coastal barriers, rocks or reefs, or simply in estuaries. Typically one or more rivers will discharge fresh water and clay minerals onto the tidal flat. Combined with tidal movements this results in widely varying salinities and the deposition of alternating sandier and muddier sediments. The muddy water is nutrient rich and typically shells occur in large numbers, albeit of relatively few species. Many move easily through the sediment. Some never emerge at the surface (typically the filter feeders), others move around on the surface or slightly below it (typically the predators).

On tidal flats locally sea-grass fields may form. These grasses attract specialised species which either feed on the grass or live in the more stable soil between the grass. When these specialised species are found in the fossil record they indicate the previous presence of sea-grass.

Marine. This comprises a wide range of environments of which only the shallowest (down to say 30 m water depth) can be sampled by the average naturalist. In the fossil record, however, deposits formed at greater depths are common and often within easy reach.

Rocky shores attract large numbers of organisms some of which attach themselves (e.g. oysters cement themselves to rocks, whereas mussels attach themselves with byssus threads) whereas others have developed strong muscles with which they can hold on to the rocks to avoid being washed away by splashing waves or by predators (e.g. Patella, Nerita). Many of these are highly specialised and only occur in a very narrow tidal range, e.g. the Littorinidae which always occur in narrow bands in the splash zone.

Others hide in fissures or under stones. It is worthwhile turning stones in the tidal or subtidal zones to see what animals live underneath, for example Diodora, Noetidae, but always make sure the stones are returned to their original position without crushing the animals (or your fingers).

A more extreme form of hiding is performed by the molluscs that perforate rocks: especially the Pholadidae. They have shells with a special structure that allows them to slowly rasp sandstone or limestone rock. After they die opportunists of non-boring species may occupy the hole that was made, e.g. Paphia, Irus and others. As a result there may be several bivalves belonging to very different species inside one another within a single hole.

Tidal pools are other hide outs with many small species living between seaweed. Washing seaweed in a basin with fresh water is a good method to sample these animals that are often too small to be seen with the naked eye.

The highest biodiversity is found on coral formations. In geological terms an important differentiation is made between coral reefs (which break the waves – and therefore occur in shallow waters) and coral banks (which do not break waves as they occur at greater depths).

Coral reefs are mostly found near rocky coasts and can form extended fringing reefs or smaller patches. In NW Borneo they are restricted to Sabah with the southernmost fringing reefs along the coast of Labuan Island and a few smaller reefs in between Labuan Island and Brunei.

Behind these reefs frequently lagoons are found, which may comprise a wide variety of biotopes including corals, sand banks, tidal flats, beach. The beaches in coral lagoons and along coral dominated coasts comprise a different mollusc fauna from those of wave dominated coasts – for example Atactodea striata, Donax cuneatus, Donax faba and others.

Coral banks occur in a much larger area including along the sandy coasts of Sarawak from water depths of a few meters to several tens of meters. The deepest coral banks are typically very flat and dominated by slowly growing corals (flat branching corals, soft corals and fan corals) whereas the shallower parts have a greater diversity including massive corals that can grow metres high).

The large variety in molluscs in coral reefs and banks is due to the wide offer in substrates and food – many live in coral sand (bivalves and carnivores from the Epitoniidae, Naticidae, Nassariidae, Mitridae, Costellariidae, Olividae, Conidae, Turridae, Terebridae), others feed on algae (Cypraeidae), live attached to or between the branches of hard corals (Arcidae, Pectinidae, Ostreidae), drill into hard corals (Pholadidae), get encapsulated by them (Pedum), or live in symbiosis or as parasites on fan corals (Ovulidae), sponges (Triphoridae), sea urchins or star fish (e.g. Eulimidae). The largest diversity is found in micromolluscs which are easiest collected by taking some samples of coral sand. A sample of 1 litre of sand from a coral bank in Sarawak typically contains hundreds of species, including more than 30 species of Triphoridae – which explains why I still have not finished sorting all the samples I took many years ago.

It is amazing how the scenery changes when the sun sets. During night dives different animals dominate coral areas: numerous fish will sleep whereas sea urchins and starfish emerge in large numbers and also many carnivorous gastropods will emerge from the sand.

Many molluscs not only specialise in food and substrate but also have a very limited depth range, e.g. Lambis lambis typically is only found on coral reefs, whereas L. scorpius and L. chiragra occur on coral banks.

A large part of the coast is formed by sandy shores, which vary from the beach to deeper parts of the shelf. A small number of molluscs lives on the beach itself, locally huge banks of Donacidae, in many places Oliva oliva which forms its characteristic trails, some Naticidae, the occasional Anadara and a wide variety of other animals that typically live in slightly deeper waters. The number of species is much larger in the subtidal areas which range from foreshore to shelf. The species living in shallow water will frequently wash up on the beach – especially after a storm also living animals can be beached. Small gastropods rarely end up on the beach as their shells are re-used by hermit crabs. It is well worth it reviewing the large number of hermit crabs that occasionally gather near the low tide line – many species I have only ever seen because hermit crabs brought them in.

There are also molluscs that live in open water and swim or float. Upon their death their shells can turn up in different depositional environments, e.g. the internal skeletons of Sepiidae (squids) float and therefore frequently wash up on the beach, as do the shells of Janthina which is a shell that lives upside down underneath the air/water interface. Also animals that live in or on floating objects (e.g. the wood boring Teredinidae) will frequently wash up on the beach, still inside or attached to these objects. The shells of animals living in plankton (such as Atlantidae, Limacinidae) will typically fall onto the seabed – they are most abundant in sediments that form slowly, e.g. the limestone sands on coral banks.